Various kinds of automated attacks are possible on web servers that provide web services, such as using stolen credentials to fraudulently access the service, brute-force attacks that try several username and password combinations to gain access, registering fake accounts, scraping websites to harvest web data, and others. Such velocity attacks typically require a large number of transactions with the web service in a very short period of time, and commonly-used web browsers are prohibitively slow for such large-scale and high-speed transactions. Instead, attackers use a wide variety of attack tools, ranging from simple shell scripts to sophisticated custom tools designed to speed up transactions.
Unfortunately, attack tools are often designed to deceive a web service into believing that the traffic is actually originating from a prevalent web browser. To achieve this subterfuge, the User-Agent header of a well-known browser is typically forged in the hypertext transfer protocol (HTTP) request header of traffic originating from a malicious attack tool. Because the User-Agent string exactly matches one of the well-known web browsers, the web service and any attack-prevention techniques that rely on identifying the User-Agent string are unable to differentiate between a real web browser and a forgery, leaving the web service vulnerable to exploitation by malicious individuals employing attack tools to access the service.